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Adequate levels of vitamin D are vital in preserving the immune system's ability to dispose efficiently of cells showing signs of early malignancy As November begins and the clocks have gone back, not only is it the time for protecting vulnerable plants from frost but also for reviewing the link between sunlight and health.

From the end of October to March the sun is too low in the sky for its ultraviolet rays to provide the necessary stimulation to the skin to make vitamin D. Like the hoarded nuts of squirrels, vitamin D stores are laid down in summer to last the winter. Even in summer, clouds remove 50 per cent of the UVB, and in winter there is little synthesis of vitamin D in the skin.

The biochemistry of the metabolism of vitamin D is hugely complex. Skin colour can be of importance in the prevention of diseases related to the lack of vitamin D. About 30 years ago research indicated that five minutes of summer sun at midday increased vitamin D levels in white people sixfold, but had no measurable effect on those with black skin.

Vitamin D deficiency is also increased by other factors. The liberal use of sunscreens can protect against skin cancer, but if overused it can reduce to inadequate levels the valuable vitamin D manufactured in human skin.

Having avoided skin cancers, the sun-wary might then find themselves in the orthopaedic clinic suffering from osteoporosis or attending clinics dealing with breast, prostate and possibly bowel cancer.

Professor Tim Oliver, emeritus professor of oncology at St Bartholemew’s and the Royal London Hospital, suspects that adequate levels of vitamin D are necessary to preserve the immune system’s ability to dispose efficiently of cells showing signs of early malignancy.

Professor Oliver, whose medical career before he switched to treating cancers was as an immunologist, has drawn attention to vitamin D defiency within immigrant families from South Asia. They share a common Caucasian ancestry with Europeans, but whereas Europeans needed all the vitamin D they could get to compete with long, dark winters, those living in India and Pakistan acquired, over time, a genetic variant of the vitamin D receptor that responded to stronger sun than the British skies provide.

The vitamin D receptor variant gene might protect South Asians living on the Indian subcontinent from renal failure resulting from vitamin D poisoning, but it can be too feeble to give those living in Britain an adequate level of vitamin D.

When Professor Oliver was an immunologist he was interested in the effect of macrophages, the cells in the immune system whose role is analogous to that of the Marines in the military. Without an adequate supply of vitamin D levels, macrophages may be unable to fight the infections and invading malignancies that are associated with the diseases of poverty.

Every GCSE student has heard of vitamin D, the sunshine vitamin. Their parents, or grandparents, will have been brought up with a knowledge of — and a firm faith in — the other common source of vitamin D, oily fish. Even during the war, when fish supplies were scarce, fish oil was still provided free for all schoolchildren. Without adequate vitamin D, children develop rickets, women (and later in life men) develop osteoporosis and the immune system works at a disadvantage.

Vitamin D is essential for the absorption and retention of calcium. Although important for bone formation, it has other effects: for instance, on the secretion of insulin and the synthesis and secretion of thyroid and parathyroid hormones. In the immune system it influences the action of T and B lymphocytes and the formation of the monocytes, all of which are white cells.

There are two types of vitamin D: vitamin D3 cholecalciferol — which is the naturally occurring vitamin — and vitamin D2, which is used in food enrichment and fortification. Vitamin D1 was an early, crudely manufactured form, now little heard about. The general view is that natural vitamin D is the first choice.

People seeking a natural source of vitamin D should take fish oil capsules or eat fresh fish. Vitamin supplements, although less easily absorbed, are readily available. A word of warning, though. Cod liver oil is also a rich source of vitamin A, but too much vitamin A must be avoided, particularly during pregnancy.

Sources of vitamin D in fish

Apart from sunlight, fish is the best source of vitamin D. Fish should be bought from non-contaminated areas of the sea. The best sources (numbers represent micrograms per 100 grams) are:

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